As conventional recording media for recording digital data, optical recording media (discs) represented by CDs (Compact Discs) and DVDs (Digital Versatile Discs) have been widely used. The optical recording media have widely employed a method of recording data in which data to be recorded is modulated into a length of recording marks along tracks. Additionally, these optical recording media are fabricated such that their various asfabricated (initial) characteristics (electrical and mechanical properties) comply with predetermined specifications, and in particular, their playback jitter values are required to be equal to or less than a certain value as a fundamental property.
One of the factors responsible for variations in playback jitter value is a variation in the power of a laser beam during recording operations.
An insufficient amount of power of the laser beam being supplied during recording operations would not accurately form marks of predetermined lengths, thereby causing an increase in playback jitter value. On the other hand, an excessive amount of power of the laser beam would cause an increase in jitter value due to deformation or the like in recording marks.
This will be explained below in more detail. To read data when the aforementioned recording method is employed, an optical recording medium is irradiated with a laser beam along the tacks to detect the reflected light, thereby reading information carried by the recording marks. On the other hand, to record data, the optical recording medium is irradiated with a laser beam along the tracks to form recording marks having a predetermined length. For example, DVD-RWs (Rewritable) or a type of the user data-rewritable optical recording medium employ recording marks having lengths corresponding to 3T to 11T and 14T (where T is one clock cycle), thereby recording data.
In general, when data is recorded on an optical recording medium, the optical recording medium is not irradiated with a laser beam that has the same pulse width as the duration corresponding to the length of a recording mark to be formed but with a laser beam of a train of the number of pulses determined in accordance with the type of the recording mark to be formed, thereby forming recording marks having a predetermined length. For example, to record data on the aforementioned DVD-RW, pulses as many as n−1 or n−2 (where n indicates the type of recording marks and takes on any one of 3 to 11 and 14) are successively impinged thereon, thereby forming any one of the recording marks having lengths corresponding to 3T to 11T and 14T. Accordingly, for n−2, to form a recording mark having a length corresponding to 3T, one pulse is used, while to form a recording mark having a length corresponding to 11T, nine pulses are used. Furthermore, for n−1, to form a recording mark having a length corresponding to 3T, two pulses are used, while to form a recording mark having a length corresponding to 11T, ten pulses are used.
To form such recording marks, it is necessary to set the laser beam to appropriate recording power for each target optical recording medium. However, the recording power provided by a laser beam may be greatly different from a desired level due to manufacturing variations of the semiconductor laser emitting the laser beam, or may be out of an appropriate range due to variations caused by its service environments or the like. In these cases, recording marks cannot be formed in an appropriate shape, resulting in jitter value being significantly degraded.
That is, although a countermeasure of controlling the radiation power provided by a laser beam within the range of predetermined levels has been taken, a problem still exists in practice that variations resulting from the manufacturing of the semiconductor laser employed or variations in its output power as well as its service environments may cause an excessive output power of a laser beam during operation, thus resulting in an increase in playback jitter value.
On the other hand, in recent years, there is an increasing demand for further improved data transfer rates of the optical recording medium. To meet this demand, the linear speed of recording/reproduction operations can be effectively increased, for which it is necessary to increase clock frequencies. However, since the laser driver for driving the semiconductor laser is limited in its operating speed, shortening one clock cycle (T) by increasing the clock frequency would raise a phenomenon that the pulses of the laser beam are limited in amplitude, thereby causing the next pulse to arrive before the power of the laser beam has been lowered to a sufficient level. Such a phenomenon would provide the same condition as setting the laser beam to a higher recording power, thereby resulting in significant degradation in jitter value.
In this context, it is desirable to form recording marks in an appropriate shape even in the aforementioned cases to provide good jitter.
The present invention was developed in view of the aforementioned conventional problems. It is therefore an object of the present invention to provide an optical recording medium which can minimize an increase in playback jitter value even in the presence of considerable variations in the power of a laser beam employed, caused by variations resulting from the manufacturing of a semiconductor laser emitting the laser beam or variations in output power of the laser beam.
It is also another object of the present invention to provide an optical recording medium which has reduced degradation in jitter when data is recorded at high data transfer rates.